GIACIATION OF LOWER COPPER RIVER 461
and the channel was deepened 12 feet in some portions and filled 12 feet in others between August, 1908, and August, 1910, in this case clearly as a result of interference with the current by the piers of the bridge.
Six miles farther to the northeast where the railway crosses the eastern distributary of Copper River from Long Island to the eastern bank, there is a main stream, Hotcake Channel, and several minor ones. Hotcake Channel is crossed by a 200 and a 400 foot steel bridge and an intermediate trestle. This channel, 15 miles from the ocean, is from 1 to 8 feet deep at low water, the low water stage being 81 feet above mean low tide at sea level and the range of river level being about 7 feet.
Throughout this portion of the Copper River delta the detailed soundings afford an especially good opportunity of watching the future variations of a glacial stream, the shifting of sand bars as revealed at low water, and the cutting and filling in various parts of the channels, as they may be revealed by soundings near the railway bridges.
Coarse Deposits of the Glacial Streams on the Delta. The surface material in the out-wash deposits of Copper River delta has ah-eady been spoken of as gravel, sand, and silt or clay. The same materials are found for a considerable depth below the surface. At the Miles Glacier bridge, for example, the river is in a 50 foot gorge and borings extend to a depth of 40 to 50 feet below the bed of the Copper River, revealing such a section as the following, ha which the exposed cliff on the south bank of the Copper River supplies the upper 56 feet and the sinking of the caisson for Pier 2 of the railway bridge the remainder.
SECTION or COABSE GLACIAL OUTWASH DEPOSITS AT MILES GLACIER BRIDGE
Material Thiclcness in Feet
Bowlders, gravel and sand 1 Large bowlders Large bowlders, lying in cemented sand, gravel and silt Large bowlders in loose sand and gravel Occasional bowlders in loose sand and gravel Bowlders in loose sand and gravel Coarse, heavy sand and gravel Compact sand and gravel, growing more compact at bottom 56 3 11 4 S 2 3 6
Total thickness revealed 88
Borings below the bottom of the river at the railway bridge across the Flag Point Channel reveal the following sections, all thicknesses being given in feet. All these deposits except the first in the left-hand column (sand and gravel, 18 feet) are below the high water surface of the river.
1 From the. surface of the plain of out-wash gravels at Milea Glacier railway station to the surface of the gravel bar in the middle of Copper River.